The present invention is generally directed to circuits utilized to feed direct current to a telephone line. The invention is more specifically directed to the minimization of noise associated with battery feed circuits.
A variety of battery feed circuits have been utilized to provide direct current (DC) over tip and ring telephone lines to customer premises equipment. Typically, a bank of DC batteries have been connected to provide a source of direct current carried by telephone lines to subscriber equipment. In addition to supplying DC, it is the purpose of a battery feed circuit to appropriately couple the audio signals transmitted from central office equipment to a subscriber and from the subscriber to the central office equipment. Thus, the battery feed circuit must take into account the need to supply a DC current to customer premises equipment and the need to couple audio between the customer premises equipment and central office equipment.
It is desirable to minimize noise on subscriber lines in order to create a quiet background for the transmission and reception of a voice signal. Various attempts have been made to minimize common mode interference signals such as by the use of differential amplifiers. Examples of such techniques are described in U.S. Pat. Nos. 4,007,335, 4,476,350, and 4,612,417.
Although improvements have been made in battery feed circuits, relatively expensive filters using large capacitors and resistors connected to the battery are utilized to minimize noise associated with the DC battery. Battery noise filtering is specially important in passive battery feed circuits, using a transformer or resistors for providing a DC feed, because there are no other means of attenuating battery noise signal. Such noise may be transferred to the battery by equipment being utilized to maintain the charge of the battery or may be induced onto leads connecting the battery to the various battery feed circuits or caused by switching equipment. For example, an undesired, signal of 60 hertz induced by commercial AC power utilized in the United States may be a component of such noise.
The tip line is normally connected through a resistor to the positive terminal of the battery which is also coupled to an earth ground. The negative terminal of the battery is coupled through a resistor to the ring line. Thus, noise associated with the battery and the leads associated with the battery is not induced as a balanced common mode noise signal which is addressed by the circuitry in the above referenced patents. Therefore, such undesired, unbalanced signals remain a problem in battery feed circuits.